In fuel injection type engines, engine speed is usually stabilized by increasing a basic fuel injection amount during warmup period immediately following startup in which speed is generally unstable.
However, as the correction coefficient is set for a fuel type having predetermined properties, it may not be suitable when a different fuel type is used, and this adversely affects drivability and exhaust gas emissions.
Tokkai Hei 3-61644 published by the Japanese Patent Office in 1991 proposes that the correction coefficient should be increased based on the engine speed. This method concerns the decline of engine speed during warm-up which occurs when fuel having a high evaporation temperature is used. Specifically, a correction coefficient is set based on standard type fuel, and when the difference between a current engine speed and target engine speed exceeds a predetermined limit, another correction coefficient is applied to increase the fuel mount.
Tokkai Hei 3-26841 published by the Japanese Patent Office in the same year discloses how it is determined whether a fuel is lighter or heavier than a standard fuel type based on a variation of engine speed a predetermined time after start-up, and corrects a fuel injection amount based on the determination result.
However in all of these methods, as the properties of the fuel are determined from the variation of engine speed each time the engine starts, some time is required until the result of the determination is known. Consequently, although the actual properties of the fuel may be different from the initial setting, an air-fuel ratio based on the initial setting is applied until this results is known, and the vehicle is therefore driven with an air-fuel ratio unsuited to the fuel type during this period. A suitable air-fuel ratio is applied only during the short time period from when the fuel injection amount is corrected based on the determination result until warm-up is completed.
Further, a transient correction applied to acceleration and deceleration during warm-up is also set for a standard fuel type. A transient correction unsuited to the fuel used is therefore applied to acceleration and deceleration before the fuel type has been determined.
In view of the above, if drivability of the vehicle is a primary consideration, it is desirable to set the post-warmup correction coefficient and the transient correction coefficient for the heaviest fuel that might be encountered. In this case however, if the fuel is light, fuel will be oversupplied, exhaust gas emissions will be worse and fuel consumption will increase until the air-fuel ratio has been corrected based on the actual fuel type used.